1. Technical Field
The present invention relates to seal designs for solid polymer electrolyte fuel cells.
2. Description of the Related Art
Fuel cells are devices in which fuel and oxidant fluids electrochemically react to generate electricity. A type of fuel cell being developed for various commercial applications is the solid polymer electrolyte fuel cell, which employs a membrane electrode assembly (MEA) comprising a solid polymer electrolyte made of a suitable ionomer material (e.g., Nafion®) disposed between two electrodes. Each electrode comprises an appropriate catalyst located next to the solid polymer electrolyte. The catalyst may be, for example, a metal black, an alloy, or a supported metal catalyst such as platinum on carbon. The catalyst may be disposed in a catalyst layer, and the catalyst layer typically contains ionomer, which may be similar to that used for the solid polymer electrolyte. A fluid diffusion layer (a porous, electrically conductive sheet material) is typically employed adjacent to the electrode for purposes of mechanical support, current collection, and/or reactant distribution. In the case of gaseous reactants, the fluid diffusion layer is referred to as a gas diffusion layer. If a catalyst layer is incorporated onto a gas diffusion layer, the unit is referred to as a gas diffusion electrode.
For commercial applications, a plurality of fuel cells are generally stacked in series in order to deliver a greater output voltage. Separator plates are typically employed adjacent the gas diffusion electrode layers in solid polymer electrolyte fuel cells to separate one cell from another in a stack. Fluid distribution features, including inlet and outlet ports, fluid distribution plenums and numerous fluid channels, are typically formed in the surface of the separator plates adjacent the electrodes in order to distribute reactant fluids to, and remove reaction by-products from, the electrodes. Separator plates also provide a path for electrical and thermal conduction, as well as mechanical support and dimensional stability to the MEA.
In an assembled fuel cell, the porous gas diffusion layers in the MEA must be adequately sealed at their periphery and to their adjacent separator plates in order to prevent reactant gases from leaking over to the wrong electrode or to prevent leaks between the reactant gases and the ambient atmosphere surrounding the fuel cell stack. This can be challenging because the MEA is typically a relatively large, thin sheet. Thus, a seal may be needed over a significant perimeter, and a fuel cell stack typically involves sealing numerous MEAs. The design of the MEA edge seal should provide for production in high volume and for reliable, high quality leak tight seals. Various ways of accomplishing this have been suggested in the art.
One such sealing method involves the use of a sealing gasket which surrounds the MEA, and which can be significantly compressed between the anode and cathode separator plates in order to effect a reliable seal between the MEA and ambient. A seal separating the anode from the cathode can be obtained by impregnating gasket seal material into the edges of the MEA and attaching or integrating these impregnated edges to the surrounding gasket. U.S. Pat. No. 6,057,054 discloses such an embodiment using flush-cut MEAs in which the edges of the membrane electrolyte, electrodes, and gas diffusion layers are aligned and terminate at the same location (i.e., at the flush cut edge). However, such an approach generally requires the same material to be used for edge impregnate as well as the gasket, and further can require tight tolerances and hence production difficulties.
Alternatively, a frame may be applied to the edge of the MEA which, in turn, is sealingly attached or bonded to the surrounding compressible gasket. In this embodiment, the electrolyte in the MEA typically extends slightly beyond the edges of the anode and cathode. The frame employed typically comprises two thin pieces applied to the edges on either side of the MEA. The frame pieces have little compressibility and essentially seal to the edge of the membrane electrolyte, thereby separating the anode from the cathode. EP1246281 discloses such an embodiment in which a frame is bonded to a surrounding, significantly compressible elastomer gasket (e.g., 1 mm thick polyisobutylene).
Other sealing methods employ more than one compressible gasket to effect the required seals. For instance, embodiments employing framed MEAs have been suggested in which the frames are not bonded to a surrounding single gasket, but are instead sandwiched between two surrounding compressible gaskets. Thus, one surrounding gasket seals an anode between the anode frame and the adjacent separator plate, while the other surrounding gasket seals the cathode between the cathode frame and its adjacent separator plate. Difficulties can arise, however, if the opposing gaskets are out of alignment with respect to each other, and tight tolerances are again required. Still further embodiments have been suggested which employ two compressible gaskets that do not employ frames on the MEAs in order to effect the desired seals. For instance, U.S. Pat. No. 6,815,115 discloses various embodiments in which one compressible gasket seal is made directly to the membrane electrolyte in the MEA, while the other gasket seal is used to make a seal between the edges of the adjacent separator plates. Here, the two gaskets are offset and so misalignment is not as much of an issue.
In all these prior embodiments, a sufficiently compressible, compliant seal is employed to seal both the anode and the cathode from the surrounding environment. However, in order to increase power density, attempts continue to be made to reduce the thickness of the individual cells making up a fuel cell stack. As fuel cell makers successfully reduce the thickness of the other components in the cells, the seal design now represents a significant limitation on further reductions in thickness. Consequently, there remains a need in the art for improved sealing methods and designs. The present invention fulfills this need and provides further related advantages.